Question

3. A man’s leg was crushed between a car bumper and a wall. His physicians believe their patient has suffered kidney damage from myoglobin blocking glomerular pores. Tests showed the following results:

                                Plasma creatinine                                            38.7 mg/dL plasma

                                24 hr urine specimen                                     volume = 1 liter

                                Urine creatinine                                                293 mg/dL urine

A. How many mg of creatinine are in the urine specimen? How much creatinine appears in the urine per hour?

                B. What is the patient’s creatinine clearance in mL/min?

                C. What is the patient’s GFR?

                D. Is this a normal GFR? Did the patient likely sustain kidney damage?

Answer 1

• The patient excretes one litre of urine. The creatinine concentration is 293mg/dL. 1litre= 1000mL = 10dL. Therefore, by basic math, there are 293 x 10 mg= 2930mg of creatinine in the urine. In 24 hours, 2930mg of creatinine appears in the urine. Thus, in one hour, 2930/24 = 122.08mg of creatinine per hour.
• I have attached the calculation as an image. ( Adobe sketchbook on ipad pro)
• Creatinine clearance test is performed to estimate the GFR of the person. It usually exceeds the GFR by 10-20%. Several formulae lay down the relationship between GFR and ClCr. GFR= ClCr x body surface area(m^2)/1.73m^2.. The BSA of the patient, if assumed to be 1.73 sq mtr, the GFR will be the same as Creatinine clearance.
• No, this is far below the lower limit of normal GFR. The patient has clearly sustained kidney damaged due to the crush injury to the muscles. The breakdown of muscles releases myoglobin which is a muscle protein. Studies indicate that thr heme in myoglobin causes lipid peroxidation and renal injury due to the recycling of iron from ferrous to derric to ferryl states.